The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly to capacitors for use in semiconductor devices.
Memories in which a ferroelectric film is used as the dielectric film for capacitors (e.g., ferroelectric random access memories) are advantageous in some respects. First, they are nonvolatile memories. Second, data can be written into them and erased from them at high speed. Further, data can be rewritten in them a large number of times.
The ferroelectric film used in such a memory is a Pb(ZrxTi1xe2x88x92x)O3 film (PZT film) or the like. The PZT film is sandwiched between upper electrodes, on the one hand, and lower electrodes, on the other, whereby ferroelectric capacitors are provided. The electrodes are usually made of a noble metal (precious metal), such as Pt, Ir, or Ru. Electrodes are known, which are made of an electrically conductive oxide such as IrO2 or RuO2 to have improved fatigue characteristic (i.e., resistance to write/read cycles). Electrically conductive oxide, such as IrO2 or RuO2, has high resistivety. Hence, it is proposed that a conductive oxide film and a noble metal film (e.g., Pt film) be laminated to form an electrode.
Conventional electrodes of the structures described above cannot be said to posses the characteristics that the electrodes of ferroelectric capacitors should have. The problems with the conventional upper and lower electrodes will be described below.
First, the problems with the upper electrode will be described.
Generally, a ferroelectric capacitor is manufactured by the following method.
First, a CMOS structure is formed on an Si substrate. Then an insulating film, such as Si oxide film, is formed. Thereafter, a Pt/PZT/Pt/Ti composite layer is formed on the insulating film. In the process of forming the Pt/PZT/Pt/Ti composite layer, the Pt film and Ti film are formed by means of sputtering, and the PZT film is formed by sputtering or sol-gel method.
The Pt/PZT/Pt/Ti composite layer is then patterned by means of dry etching. CVD process is performed, forming an insulating film made of, for example, silicon oxide, thereby covering the composite layer patterned. RIE process is carried out, forming via holes. Sputtering is then effected, forming a wiring layer. Further, annealing is performed in a forming-gas atmosphere, thereby eliminating process damages to the transistor.
Of the various processes mentioned above, the capacitor are greatly influenced by reducing gas in the dry etching and the forming of the insulating film. When the PZT film is exposed to a reducing atmosphere, the perovskite structure will release oxygen. This is because the structure needs to remain in thermal equilibrium or the reducing atmosphere contains active hydrogen. The oxygen vacancy in the perovskite structure decreases the polarization value and changes the electrical conductivity. Consequently, the capacitor will have insufficient ferroelectricity. The polarization-retaining property, imprint property and the like of the capacitor are insufficient, too, perhaps because of the polarization caused by the space charge generated due to the oxygen vacancy.
The imprint property results in a hysteresis shift of the capacitor when writing is performed in one direction. For example, writing is performed in one direction and keeping the written state for a long period, thereafter writing is performed in opposite direction. In this case, the polarization-retaining property of the capacitor in the opposite direction is much deteriorated.
If the upper electrode of the capacitor is made of a noble metal such as Pt, it will work as a catalyst in a reducing atmosphere, generating active hydrogen. The active hydrogen reduces the PZT at the interface between the PZT film and the Pt electrode. Therefore, the characteristics of the capacitor change. If the lower electrode of the capacitor is made of Pt, the PZT/Pt interface is heated to a high temperature (600xc2x0 C. or more) at the time of crystallizing the PZT. Ti used to a layer that bonds the lower electrode to the underlayer and Pb diffuse into the grain boundary of Pt and forms an oxide. Thereafter, the catalytic action of Pt is suppressed. Process damage can therefore be more suppressed at the lower electrode side than at the upper electrode side.
The upper electrode may be made of an electrically conductive oxide such as IrO2 or RuO2. Alternatively, the upper electrode may be made of a layer of such an electrically conductive oxide and a Pt layer provided on the oxide layer. In either case, not only the fatigue characteristic may be improved, but also the process damage developing in the reducing atmosphere may be suppressed. However, the electrically conductive oxide, i.e., IrO2 or RuO2, may be reduced. If the oxide is reduced, Ir or Ru will perform catalytic action, ultimately changing the characteristics of the capacitor as described above.
Now, the problems with the lower electrode will be described.
In most ferroelectric memory elements, the capacitors are provided in regions other than the transistor regions. Hence, the ferroelectric memory elements can hardly be arranged in high density. To enhance the integration density of the ferroelectric memory elements, the memory needs to have so-called xe2x80x9cCOP (Capacitor-On-Plug) structure,xe2x80x9d in which capacitor is provided directly on the contact plug. The COP structure is disadvantageous in that the material of the plug, either W or Si, reacts with PZT. An oxide of W or Si can be easily formed. The oxide, if formed, increases the contact resistance and causes morphological deterioration. To prevent this, it is necessary to form a barrier layer of TiN, TiAlN, TiSiN, TaN, TaSiN or the like on the plug made of W or Si.
If a barrier layer is formed, its elements may diffuse into the PZT film. Even if an electrically conductive oxide, such as IrO2 or RuO2, is formed on the barrier layer, the oxide can hardly inhibit the elements from diffusing into the PZT film. Furthermore, Ir or Ru in the electrically conductive oxide may diffuse into the PZT film to react with the elements of the PZT film. If Ir or Ru diffuses into the grain boundary in the PZT film, an electrically conductive oxide will be formed at the grain boundary. The oxide increases the leakage current that flows through the PZT film.
Thus, the diffusion of an element from the material of the lower electrode into the PZT film will be a great problem with the lower electrode.
As indicated above, a capacitor having a dielectric film (PZT film or the like) made of an oxide will be useful in semiconductor memories. However, the neither the upper electrode nor lower electrode of the conventional capacitor can be said to have optimum characteristic. The upper and lower electrodes of the conventional capacitor therefore change or deteriorate the characteristics of the capacitor.
The object of the present invention is to provide a semiconductor device, which comprises capacitors, each having a dielectric film made of an oxide, and in which changes or deterioration of the characteristics of the capacitors can be inhibited.
According to this invention there is provided a semiconductor device which comprises a semiconductor substrate and a capacitor provided above the semiconductor substrate and having an upper electrode, a lower electrode, and a dielectric film made of oxide and provided between the upper and lower electrodes. At least one of the electrodes comprises an SrRuO3 film provided near the dielectric film and a conductive film made of conductive material other than SrRuO3 and provided far from the dielectric film.
If the upper electrode has the structure described above, the following advantages will be attained.
SrRuO3 (SRO) is an electrically conductive oxide having perovskite structure and excels in resistance to reduction, unlike other electrically conductive oxides such as IrO2 and RuO2. Hence, the dielectric film made of oxide such as PZT will not be reduced even if a film of noble metal (precious metal) such as Pt, Ir, Ru or the like, which performs a strong catalytic action, is formed on an SRO film.
A capacitor was prepared, in which a PZT film was used as dielectric film made of an oxide, and in which a composite layer made of an SRO film and a Pt film laid on the SRO film was used as an upper electrode. When this capacitor was heated in a forming gas (i.e., nitrogen atmosphere containing 1 to 3% of hydrogen) at 450xc2x0 C., the remanent polarization decreased from the initial value by 20 to 30% only. On the other hand, when the upper electrode was made of IrO2 or RuO2, and heated in the same conditions, the PZT film scarcely exhibited ferroelectricity.
In the present invention, an SRO film is interposed between the dielectric film made of oxide and a conductive film made of a prescribed conductive material. The resultant structure can acquire high resistance to reduction, which neither an IrO2 film or an RuO2 can acquire. Therefore, PZT would not be reduced in a reducing atmosphere. The oxygen vacancy will be decreased, and the characteristics of the capacitor will not change.
If the lower electrode has the structure described above, the following advantages will be attained.
SrRuO3 (SRO) excels in barrier property in respect of metal elements and the like, unlike other electrically conductive oxides such as IrO2 and RuO2. In addition, the elements in SRO hardly diffuse. Hence, if a conductive film (which may function not only as a lower electrode but also a wire) is provided beneath the SRO film, the metal elements contained in the conductive film are prevented from diffusing into the dielectric film such as the PZT film. The characteristics of the capacitor are not deteriorated. The conductive film provided below the SRO film can therefore be made of various materials. It can be made not only of Pt, Ir, Ru and the like that have been hitherto used, but also of conductive materials such as TiN, W, TaN, silicide and the like.
Both the upper electrode and the lower electrode may have the structure described above. If this is the case, the capacitor will have an improved resistance to reduction and an enhanced barrier property in respect of metal elements.
In the semiconductor device, it is desirable that the SrRuO3 film has a thickness greater than 2.5 nm and less than 40 nm.
In the semiconductor device, it is desirable that the dielectric film is made of a dielectric material represented by formula ABO3, where A is at least one A-site element, B is at least one B-site element, and O is oxygen, or a dielectric material represented by formula Bi2Axxe2x88x921BxO3x+3, where A is at least one A-site element, B is at least one B-site element, Bi is bismuth, and O is oxygen.
According to the invention there is provided a method of manufacturing a semiconductor device which comprises a semiconductor substrate, and a capacitor provided above the semiconductor substrate and having an upper electrode, a lower electrode, and a dielectric film made of oxide and provided between the upper and lower electrodes, and in which at least one of the electrodes comprises an SrRuO3 film provided near the dielectric film and a conductive film made of conductive material other than SrRuO3 and provided far from the dielectric film. The method comprises the steps of crystallizing an amorphous film of the material of the dielectric film, thereby forming the dielectric film, and crystallizing an amorphous film of the material of the SrRuO3 film, thereby forming the SrRuO3 film.
According to the invention there is provided another method of manufacturing a semiconductor device which comprises a semiconductor substrate, and a capacitor provided above the semiconductor substrate and having an upper electrode, a lower electrode, and a dielectric film made of oxide and provided between the upper and lower electrodes, and in which at least one of the electrodes comprises an SrRuO3 film provided near the dielectric film and a conductive film made of conductive material other than SrRuO3 and provided far from the dielectric film. The method comprises the step of forming a film between the dielectric film and the SrRuO3 film, wherein the film contains at least one B-site element of a dielectric material represented by formula ABO3, where A is at least one A-site element, B is at least one B-site element, and O is oxygen, or at least one B-site element of a dielectric material represented by formula Bi2Axxe2x88x921BxO3x+3, where A is at least one A-site element, B is at least one B-site element, Bi is bismuth, and O is oxygen.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.